How to make a TreeView?

I tried to use the following: ...and just spotted that: The problem is, that my project is started with Bootstrap 4.

What can I do now ? Is there a way to change the Bootstrapversion of the project afterwards?

Well you certainly don't want to go backwards from BS4 to BS3 just to use a 3rd party sinppet.

Here's a tree using pure CSS which will work in BS4

Here are some more options